Various materials exist for which a minimal intrinsic attenuation of the order of 10.sup.-2 to 10.sup.-4 dB/km can be predicted in the spectral region comprised between 2 and 12 .mu.m. They are therefore considered to be suited to the fabrication of extremely low loss optical fibers to be used for transmission systems with widely-spaced repeaters, operating in the medium infrared. Yet, materials used to manufacture optical fibers must have various characteristics not only of the optical type: namely high mechanical resistance, chemical and structural stability, low reactivity with the environment.
Among the various materials those, whose characteristics more strictly satisfy these requirements, are halide glasses and, more particularly fluoride and chloride glasses having metal fluorides and chlorides as basic compounds.
Even though glass structures derived from elements of Group II of the Periodic Table (Be, Zn, Ba) or of Group III (Al, Sc, La, Th) may be used, matrices derived from the elements of Group IV (Hf, Zr) have proved to be particularly suited for the optical transmission in the medium infrared, ranging from 2 to 8 .mu.m. Fluorohafnate and fluorozirconate glasses, discovered in France in 1976, are in common use and have all the characteristics necessary for a material to be used in the optical telecommunications field.
Chloride-based glasses have also been lately manufactured and their basic properties are now being investigated. See e.g. the paper entitled "Cadmium halide glasses", di M. Mateki et al, The Journal of Non-Crystalline Solids, 56 (1983), pages 81-86.
These glasses are also very promising since they present a minimum attenuation value at wavelengths higher than those of fluoride glasses. Namely chloride glasses have an attenuation minimum in the 6 .mu.m region, while the attenuation minimum of the fluoride glass is in the neighborhood of 3 .mu.m.
Hence the chloride glasses have a wider optical pass-band and consequently a lower attenuation value.
Optimal performances can be obtained by the use of glass manufacturing processes ensuring ultra-high purity levels and perfect optical guide structure.
These requirements can only be met by processes very similar to the well known chemical vapor phase deposition (CVD) or anyway by processes exploiting the synthesis of liquid or vapor phase reactants. In these states, in fact, a reactant can attain a very high purification degree and the reaction can take place at temperatures much lower than those required of solid reactants, thanks to the close contact between the parts, ensured by the fluid state.
A low temperature reaction is to be preferred not only for economical reasons, but also because it permits a separation between the reactive phase and the phase in which the optical fiber guide structure is built up.
In fact, if the reaction certainly occurs below a certain maximum temperature, the temperature of the first treatment of the solid particles produced can be selected at will.
One can thus operate under the best matrix vitrificability conditions. Besides, at low temperature the space arrangement of the produced material can be maintained, thus avoiding the rise of perturbations in the guide structure.
Such a space arrangement is still maintained thanks to the low-reaction temperature, as gaseous products are evacuated without giving rise to turbulence in the reaction mass.
The optical fibers produced with multi-component fluoride or chloride-based glasses can reach minimum attenuation values of the order of 10.sup.-3 dB/km and hence they can be used in ultra-long distance connections, e.g. they can be installed in transatlantic cables with very few if any intermediate repeaters.
Of course, it is advisable to provide very long fiber trunks, in order to reduce as much as possible the number of splices. In fact fiber splices introduce minimum attenuation values of about 0.1 dB; that is why each splice produces the same attenuation of about 100 km of optical fiber made of the glasses described.
Hence processes of continuous fiber production are required, i.e. processes wherein glass production, preform manufacture and drawing can be effected without interruption.
Processes of this kind are already known, even though the used compounds are generally solid or the production of glass in powder state is effected off line.